The Capital One skill allows you to check your credit card balance or make a payment when one is due. This is secure: The skill performs security checks and requires you sign in using your username and password. Then, when you open the skill, you must provide a four-digit code to confirm your identity. Just be wary of who is around when using the skill -- anyone who overhears you say your personal key can access your banking or credit card info just by asking Alexa.
Like all AI devices that use National Language Processing or NLP, Alexa does not understand all voices easily. But she learns to understand her people over time. She does, however, require people to speak in simple terms, with appropriate pauses, and use specific word orders. There are many times when she will say "I'm sorry, I don't know the answer to that question." More than likely, she doesn't recognize a word order or misunderstood the question.